February 20, 2025

Jessica A. Dennehy 

11th Grade

Williamsville East High School 

Introduction

The oceans consist of over 70% of Earth's surface, with humanity having explored only 5% of it. As a result, vast regions like the Twilight Zone—the dimly lit region spanning from 650 to 3,300 feet beneath the surface—remain largely shrouded in mystery. 

Sandwiched between the epipelagic zone above and the bathypelagic zone below, the twilight zone remains one of the least explored environments on the planet. Also referred to as the mesopelagic or midwater zone, this is the region where sunlight fades to perpetual near-darkness. It also contains the thermocline—a critical layer where water temperature decreases rapidly with depth; however, the depth and intensity of the thermocline vary with seasons and geography, being strongest in tropical waters and nearly absent during polar winters. Early scientists once believed that the extreme conditions of the twilight zone—near-freezing temperatures, crushing water pressures of up to 1,500 pounds per square inch, and complete darkness—were too harsh to sustain significant life. Without sufficient light for photosynthesis, the primary producers that fuel most ecosystems cannot survive in these depths. However, modern discoveries have proven this to be false, revealing that the twilight zone is not only teeming with life but also vital to the health of global ecosystems. By playing a crucial role in nutrient cycling and carbon storage, this watery expanse has been transformed into a vibrant frontier of scientific exploration, filled with countless mysteries simply yet to be uncovered.

Due to the extreme living conditions of the Twilight Zone, its inhabitants have adapted to the inky depths. Aside from bottom dwellers, about 90% of all animals living below 300 feet can produce their own light—a characteristic known as bioluminescence. The emitted light can then be used for a variety of purposes: camouflage, attracting mates, scaring off predators, or even luring in prey. With bioluminescence being such an important trait in the Twilight Zone, adaptations such as large eyes that are able to pick up even the slightest sliver of light penetrating the inky blackness are widely prevalent. 

In addition to harsh physical conditions, deep-sea animals have also adapted to the scarcity of food within the twilight zone. Deep-ocean habitats are too dark to support photosynthetic organisms, such as plants, to support the rest of the food chain. Instead, the predominant food source in the twilight zone is actually debris from shallower waters, called marine snow. These bits and pieces of organic matter rain down from more productive waters, serving as the predominant food source for the overwhelming majority of deep-water species. Those who don’t scavenge in the snow may make daily commutes up and down the water column to feed in closer to the surface. Traveling thousands of feet each way to feed every night, these species always return to deeper waters by morning. Many biologists believe that these vertical commutes potentially are the greatest mass migrations on Earth. With the difficulties of living in the Twilight Zone come impressive adaptations to its extreme conditions, with different animals contending in different means for survival in the deep.

The Anglerfish

In the absence of sunlight, a necessary component for photosynthesis, deep-sea exploration has uncovered remarkable ecosystems that rely on alternative energy sources. In places like Monterey Bay's submarine canyon, cold, sulfide-emitting seeps, along with their plethora of sulfur-eating bacteria, support the entirety of the ecosystem. These bacteria serve as substitutes for the photosynthetic producers found in surface-level food chains. Forming symbiotic relationships with the likes of clams and tube worms, the ecosystems of the seeps are only possible due to their bacterial backbone. Similarly, hydrothermal vent ecosystems rely on the energy of heated, chemical-rich waters emitted from the seafloor. These vents spew water sometimes as hot as molten lead, along with a variety of minerals like hydrogen sulfide. The extreme heat and chemical emissions support the communities of giant tube worms, blind shrimp, and various species of bacteria that call these deep-vent ecosystems their home—who are all capable of using sulfur or methane as energy. These bacteria form the foundation of the food chain, sustaining an array of organisms that thrive in conditions once thought impossible for life to survive.

Life in the twilight zone plays a crucial role in regulating Earth's climate. As twilight zone organisms navigate between the twilight zone and shallower waters, they also transport vast amounts of carbon from the sunlit surface waters into the deep. When migratory species ingest phytoplankton, along with other carbon-rich organisms in the sunlight zone, they then transport their carbon to the deep as they return to the twilight zone. Additionally, marine snow is also carbon-dense; between being eaten by deep-water scavengers and settling onto the ocean floor, this carbon can remain locked away for millennia. Trapping carbon in the deep ocean prevents it from returning to the atmosphere as carbon dioxide, thus regulating global temperatures. Each year, around 200 million tons of carbon are contained through this process, known as the biological pump. Beyond its environmental significance, the Twilight Zone remains one of the least disturbed parts of our planet. With increasing efforts to protect the region from exploitation, numerous governments and organizations are supporting initiatives to suspend deep-sea mining. Additionally, governments, including the United States, have begun enacting precautionary bans on certain twilight zone fisheries in the Pacific. There is still hope to preserve the wonders and the inhabitants of the deep.